One type of memory is resistive memory. Resistive memory utilizes the resistance value of a memory element to store one or more bits of data. For example, a memory element programmed to have a high resistance value may represent a logic “1” data bit value and a memory element programmed to have a low resistance value may represent a logic “0” data bit value. Typically, the resistance value of the memory element is switched electrically by applying a voltage pulse or a current pulse to the memory element. A set pulse changes the resistance of the element from a high resistance state to a low resistance state, and a reset pulse changes the resistance of the element from a low resistance state to a high resistance state.
Transition Metal Oxides (TMO) belong to the group of materials which can be used for non-volatile resistance switching memory elements. Such materials include, for example, NiOx, TiO2, and Cr-doped SrTi(Zr)O3. One theory explains the switching behavior of TMO memory elements by the formation and rupture of a conducting filament in the oxide matrix. According to this theory, the conducting filament is formed by the collection of oxygen vacancies at structural defects due to the applied electrical field during the set operation, and destroyed by thermal rupture during the reset.
The initial (i.e., “virgin”) resistance state of TMO memory elements is a high resistance state. A single high-voltage quasi-static sweep or a triangular voltage pulse can be applied to the memory element during an initialization phase in order to reach a switching of the element from the virgin high resistance state to the low resistance state. The high voltage initialization sweep used in the initial formation of the conducting filament typically has a much larger amplitude than that of the standard set pulses used in normal operation. This can be explained by the formation of an initial conducting filament, which is not completely destroyed during the consecutive reset but only broken up at a weak link. Therefore, the resistance of a memory element in the reset state is usually lower than that of a memory cell in the virgin state. The conducting filament can be “reformed” using a lower voltage during the subsequent set operations.
The high initial formation voltage (Vformation) for switching a TMO memory element out of the virgin resistance state is typically much larger than the set voltage pulses (Vset), and the set voltage pulses (Vset) are typically larger than the reset voltage pulses (Vreset) (i.e., Vformation>>Vset>Vreset). The high initial formation voltage amplitude used during initialization holds the risk that the memory element could get damaged by the high current flow when it switches to the low resistance (set) state, which can lead to an unreliable memory cell or even to a complete destruction of the cell. Also, the addition of high voltage circuits to a memory device for performing the initialization adds to the cost and complexity of the memory device.
For some materials, such as for NiOx films, the initial formation voltage (Vformation) can be reduced by under-oxidation of the film. However, the initial formation voltage for these NiOx memory elements can still be relatively large (e.g., approximately 50% higher than the normal set voltage (Vset). In addition, the under-oxidation of the NiOx film goes along with a reduction of the initial resistance of the material. As the initial resistance defines the maximum accessible switching ratio, the sense margin of the memory element is also strongly reduced by the under-oxidation.